1. Field of the Invention
The invention relates to a vacuum valve assembly and, more particularly, to a cluster valve for use with semiconductor wafer processing systems.
2. Description of the Background Art
Semiconductor wafer processes such as plasma etching and chemical vapor deposition (CVD) are often performed in sub-atmospheric conditions. To ensure peak performance in wafer processing, efficient operation of an exhaust assembly is required in order to pump away reaction by-products and maintain a fresh supply of reactant gases to the process chamber. FIG. 1 illustrates schematically a typical exhaust assembly 100 connected to a chamber body 101. The exhaust assembly 100 comprises for example, a vacuum coupler 102, with an isolation valve 104 and a throttle valve 106 for control of the pumping operation. Diagnostic accessories (not shown) may also be connected to the vacuum coupler 102 via a valve 108. The isolation valve 104 is used to isolate the process chamber body 101 from the pumping foreline 110, while the throttle valve 106 allows control of the pumping capacity. O-ring seals are typically used to provide vacuum sealing between the flanges of the vacuum components. The existing design of vacuum flange couplings incorporating an O-ring seal usually leaves a space between the mating vacuum flanges. This is illustrated in FIG. 1 by the gaps 112, 114 and 116 between the chamber body 101, vacuum coupler 102, isolation valve 104, and the throttle valve 106. The non-metallic O-ring material is also a poor thermal conductor. As such, there is minimal heat transfer between the process chamber body 101 and components in the exhaust assembly 100, such as the isolation valve 104 and the throttle valve 106.
In many CVD applications, the process gases and by-products are often non-volatile or readily condensable, and may result in undesirable deposits inside the chamber body 101 or the exhaust assembly 100. For example, in the deposition of silicon using a reaction of tetraethyl orthosilicate (TEOS) and ozone (O.sub.3) , TEOS and reaction by-products tend to condense onto cold interior surfaces of the exhaust assembly 100. The accumulation of these deposits leads to clogging of the vacuum components such as the isolation valve 104 and the throttle valve 106, and contributes to a deterioration of the pumping capacity and process performance. At a pressure of about 200 torr, TEOS condenses at temperatures below about 65.degree. C. Therefore, it is common practice to maintain the chamber body 101 and the exhaust assembly 100 at some elevated temperature to minimize the formation of these deposits. The chamber body 101, for example, may be heated by a resistive heater embedded in a chamber liner (not shown). In certain applications, a heater used to maintain a wafer support pedestal at an optimal processing temperature may also contribute to heating the chamber body 101. The exhaust assembly 100 is typically heated externally by heating tapes or cartridge heaters (not shown) around the various vacuum components. However, these heaters invariably add to the cost and complexity of the operation of the exhaust assembly 100. The need for external heaters for the exhaust assembly 100 can be eliminated if thermal conduction can be improved between the heated chamber body 101 and the exhaust assembly 100.
In addition to heating, the process chamber 101 and the exhaust assembly 100 are also subjected to periodic dry cleaning procedures using chlorine (Cl.sub.2) or nitrogen fluoride (NF.sub.3) gases in either thermal or plasma conditions. The throttle valve 106, being located farthest downstream in the exhaust assembly 100, may not be as efficiently cleaned as, for example, the isolation valve 104, due in part to the depletion of the reactive cleaning gas.
Therefore, there is an ongoing need for alternatives to facilitate equipment maintenance by providing a more compact design and improved thermal conduction between the chamber body 101 and the exhaust assembly 100.